1. Field of the Invention
This invention relates to a computer system and more specifically to supplying electrical power for a computer system.
2. Description of the Related Art
Computer systems in general and International Business Machines (IBM) compatible personal computer systems in particular have attained widespread use for providing computer power to many segments of today's modern society. A computer system generally includes a system processor which is coupled to associated volatile and non-volatile memory. A personal computer system can generally be defined as a desk top, floor standing, or portable computer system that further includes a display monitor, a keyboard, one or more floppy disk drives, and a fixed disk storage device. One of the distinguishing characteristics of personal computer systems is the use of a system board to electrically connect these components together. A personal computer system may also include one or a plurality of input/output (I/O) devices (i.e. peripheral devices) which are coupled to the system processor and which perform specialized functions. Examples of I/O devices include modems, sound and video devices or specialized communication devices. Mass storage devices such as hard disks, CD-ROM drives and magneto-optical drives may also be considered peripheral devices.
As the computing power of personal computer systems has grown, these personal computers systems are now used as network server systems. Network server systems manage the shared storage or processing requirements of groups of personal computer systems. An example of one such network server system is available from Dell Computer Corporation under the trade designation Dell Poweredge 4100.
One concern associated with the design of a computer system is how to reliably supply electrical power to the computer system. Several components of the computer system require power to operate and those components may require power at different voltage levels. For example, the computer system may have a Direct Current (DC) brushless motor fan that requires power at +12 volts direct current (VDC) to rotate to cool the computer system. The disk drives of the computer system may also operate at +12 VDC. The microprocessor, random access memory (RAM) and other electronic components may operate at either or both +5 VDC and +3.3 VDC signal voltages. Additionally, the computer system may require -5 VDC, -12 VDC and +5 V flea power (FP). Each of these components are connected to the power supplies via respective power nodes.
One source of power for a computer system is a battery power supply where the power for the computer is provided by batteries. Another source of power for a computer system is a power converter that converts Alternating Current (AC) to DC. One type of power converter converts alternating current from an 120 volt AC source provided to most homes and business into DC voltages at different voltage levels.
A problem in supplying power to a computer system is that a failure of a power supply may cause the computer system to crash. One way to prevent these power sources from failing is to regularly replace the power supply or add a redundant power supply.
Normally, installing a new power source to computer system requires that the computer system be shut down. One method to avoid a shut down is to install a second power supply while a first power supply is supplying power to the computer system. In this method, the second power supply is referred to as a hot-pluggable power supply as the power supply is installed while the computer system is operating. However, installing a second power source while the computer system is operating may cause voltage problems in the computer system. For example, switching a second power supply on-line that is operating at below specified voltage levels can cause the voltage level of the computer system to dip and possibly cause the entire computer system to shut down.
To remedy this problem of switching a power supply on-line, resistive loads are connected to the outputs of the power supplies. For example, referring to FIG. 1, a known system for supplying power to a computer system is shown. The system includes power supply 101 and power supply 104. Power supply 101 supplies a DC voltage to the computer system via an output terminal 107. Power supply 104 supplies a DC voltage to the computer system via an output terminal 110. At power-up, switch 124 is closed and power supply 104 supplies the computer system with power. Additionally, switch 121 is also closed and power supply 101 supplies the computer system with power. Thus, the output terminals 107 and 110 of power supply 101 and power supply 104 are coupled. Consequently, if power supply 101 is operating at a lower voltage than a specified voltage and switch 121 is closed, power supply 101 pulls the voltage level at the computer system power input terminal 127 below the specified voltage level and thus could bring the entire computer system down.
To remedy the potential problem of power supply 101 supplying power to the system below a specified voltage level, a resistive load 129 is connected to the output terminal 107 of power supply 101. Before switch 121 is closed, output terminal 107 is connected to resistive load 129 to provide an initial load. Control circuit 131 then closes switch 121 to provide additional computer system load. After the computer system load is connected, the power supplies are monitored to determine whether they are operating within specification. However, resistive load 127 remains connected in parallel with the computer system load. Consequently resistive load 127 dissipates power as heat energy which the cooling system of the computer system must remove. Resistive load 134 is coupled to power supply 104 to provide a load to power supply 104 when switch 124 is open.
One problem with the above remedy is that after the power supply is supplying power to the system, the resistive load is still connected. Thus, the resistive load is still consuming power from the power supply. Consequently, a larger power supply must be used. Furthermore, because the resistive load is being powered by the power supply, the resistive load produces heat which can shorten the working life of other computer system components. Additionally, because the power supply is not tested until after it is connected to the computer system, the power supply can cause the computer system to crash before any problem with the power supply is detected.